The demands from computer users for multimedia capability are increasing rapidly. Today's computers must integrate numerous functionalities including, e.g., CD-ROM, computerized movies, high-fidelity sound, gaming functions, and speakerphone capability. The computer's audio circuit must be able to interface with each of these sources, perform the necessary conversions between formats and sample rates, and direct the audio to the desired output. Integrating these various functionalities creates complexity and increases the expense of the audio circuit. In addition, integrating communications such as speakerphone capability with computer audio (such as .WAV files developed by MicroSoft) requires multiple coder/decoders (codecs) and various discrete components. This results in added costs, and also generates problems with respect to audible noise coupling and high frequency radiation (EMI).
One computer that integrates audio information from various sources is the Compaq Presario.RTM.. A block diagram of the audio circuit of the Presario.RTM. is shown in prior art FIG. 1. The audio circuit is centered around audio codec 11. Audio codec 11 may be, for example, the ES1888 chip made by ESS Technology. Audio codec 11 receives audio from CD audio input 13, MPEG audio input 15, modem digital signal processor (DSP) connection 17 (i.e., audio from a modem connected to a phone line for the speakerphone), and microphone 19. Serial data from connection 17 are applied to audio codec 11 and audio from microphone 19 is sent to connection 17 via an external codec 21. External codec 21 may be, for example, an AT&T 7525 codec. The external codec 21 allows the audio circuit to function as a full-duplex speakerphone. Additional inputs to audio codec 11 include line in 35, which is typically a jack on the exterior of the computer or monitor housing (not shown) that allows the user to apply audio signals to the audio circuit. Joystick 37 (or other game control device) also interfaces with audio codec 11.
Audio codec 11 communicates with ISA bus 23 via bus 25. An I.sup.2 C bus 27 is derived from bus 25 and is applied to tone control circuit 29 via interface logic 31. Tone control circuit 29 selectively processes the audio produced by audio codec 11 and applies its output to 3D sound circuit 33, as explained in more detail below. I.sup.2 C is an industry standard communication link comprising a clock signal and a data signal. General purpose I/O output one (GP1) from audio codec 11 is used to selectively bypass 3D sound circuit 33. Addressable registers of the audio codec 11 are used to control and access the 1.sup.2 C bus 27, and audio codec 11 uses a general purpose I/O output zero (GP0) to indicate decoded reads from or writes to these registers. The GP0 output of the audio codec 11 is connected to the logic 31.
Audio codec 11 generates an audio output on line 39 which applied to tone control circuit 29. Tone control circuit 29 may be, for example, the TEA6330 chip available from Phillips. By way of the logic 31, tone control circuit 29 can be controlled remotely via ISA bus 23 and I.sup.2 C communication link 27. Tone control circuit 29 generates an output on line 41, which is in turn applied to 3D sound circuit 33. 3D sound circuit 33 may be, for example, the AN7395 chip available from Panasonic. The 3D sound circuit may be selectively bypassed by activating GP1 of audio codec 11. The output of 3D sound circuit 33 is applied to audio amp 43 and to line out 45. Line out 45 is typically a jack on the exterior of the computer or monitor housing. Audio amplifier 43 amplifies the audio signal and outputs it to speaker/headphone output 47.
To reduce EMI and noise coupling within the system, all analog inputs and outputs are isolated from audio codec 11 via an EMI filter 49. EMI filter 49 may be any of the filters known in the art, including for example a pi-filter having two capacitors and a ferrite bead. Power for the audio circuit is supplied by linear regulator 51, which produces the clean 5-volt DC output necessary for analog components. The linear regulator may be, for example, the LM317SX regulator available from National Semiconductor.
A block diagram of the internal structure of audio codec 11 is provided in prior art FIG. 2. Inputs from microphone 19, CD audio 13, line in 35, and MPEG audio 15 are applied to an input multiplexor 53. Input multiplexor 53 outputs left and right analog audio signals that are applied to the left and right channels of analog-to-digital (A/D) converter 55. A/D converter 55 comprises two 16-bit A/D converters (e.g., a delta/sigma converter); one for each channel. The resulting digital left and right audio signals are applied to bus interface logic 57, which is in communication with ISA bus 23 via link 25.
Digital audio output from bus interface logic 57 is applied to digital-to-analog (D/A) converter 58. Converter 57 comprises two 16-bit D/A; one for each channel. The analog outputs from D/A converter 57 are applied to mixer 59.
An FM or wave table audio synthesizer 61 is also provided in communication with ISA bus 23. Synthesized left (60) and right (62) audio from audio synthesizer 61 are applied to D/A converter 63, which has the same structure as that of D/A converter 57. The outputs of D/A converter 63 are also applied to mixer 59. Mixer 59 mixes the left and right channels of the outputs of converters 57 and 63, respectively, and produces audio out left and audio out right signals that are then applied to tone control circuit 29, as explained previously.